Fixes duplicate/triple OS notifications on machines where more than
one browser (e.g. both Chrome and Edge on Windows) independently got
notification permission for the site — each held its own valid
subscription, so every event pushed to both.
Adds a userAgent column (captured client-side at subscribe time,
display-only) and a Settings > Account section listing a user's own
subscribed browsers with a friendly label plus an unsubscribe button,
backed by new GET/DELETE /api/push/subscriptions endpoints scoped to
the caller's own rows.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Gh2UXUQUVBWroWEnn1FLFG
